1. Field of the Invention
The present invention relates to an in-plane switching mode liquid crystal display (LCD) device. More particularly, the present invention relates to an in-plane switching IPS mode LCD device and a method for manufacturing the same for improving efficiency in hardening a sealant by photo (UV).
2. Discussion of the Related Art
Demands for various display devices have increased with development of an information society. Accordingly, many efforts have been made to research and develop various flat display devices such as liquid crystal display (LCD), plasma display panel (PDP), electroluminescent display (ELD), and vacuum fluorescent display (VFD). Some species of flat display devices have already been applied to displays of various equipment.
Among the various flat display devices, liquid crystal display (LCD) devices have been most widely used due to advantageous characteristics of thinness, low weight, and low power consumption, whereby the LCD devices provide a substitute for a Cathode Ray Tube (CRT). In addition to mobile type LCD devices such as a display for a notebook computer, LCD devices have been developed for computer monitors and televisions to receive and display broadcasting signals.
Despite various technical developments in the LCD technology having applications in different fields, research in enhancing the picture quality of the LCD device has been, in some respects, lacking as compared to other features and advantages of the LCD device. In order to use LCD devices in various fields as a general display, the key to developing LCD devices depends on whether LCD devices can implement a high quality picture, such as high resolution and high luminance with a large-sized screen, while still maintaining low weight, thinness, and low power consumption.
A LCD device includes an LCD panel for displaying a picture image, and a driving part for applying a driving signal to the LCD panel. The LCD panel includes first and second glass substrates that are bonded to each other at a predetermined interval, and a liquid crystal layer injected between the first and second glass substrates.
The first glass substrate (a TFT array substrate) includes a plurality of gate and data lines, a plurality of pixel electrodes, and a plurality of thin film transistors. At this time, the plurality of gate lines are formed on the first glass substrate at fixed intervals in one direction, and the plurality of data lines are formed at fixed intervals perpendicular to the plurality of gate lines. Then, the plurality of pixel electrodes of a matrix arrangement are respectively formed in pixel regions defined by the plurality of gate and data lines crossing each other. The plurality of thin film transistors are switched according to signals of the gate lines for transmitting signals of the data lines to the respective pixel electrodes.
The second glass substrate (a color filter substrate) includes a black matrix layer that excludes light from regions except the pixel regions of the first substrate, R/G/B color filter layer displaying various colors, and a common electrode to obtain the picture image. In the case of an IPS mode LCD device, the common electrode is formed on the first glass substrate.
Next, a predetermined space is maintained between the first and second glass substrates by spacers, and the first and second substrates are bonded to each other by a sealant pattern having a liquid crystal injection inlet. At this time, the liquid crystal layer is formed according to a liquid crystal injection method, in which the liquid crystal injection inlet is dipped into a container having liquid crystal while maintaining a vacuum state in the predetermined space between the first and second glass substrates. That is, the liquid crystal is injected between the first and second substrates by an osmotic action. Then, the liquid crystal injection inlet is sealed with the sealant.
The LCD device is driven according to optical anisotropy and polarizability of liquid crystal. Liquid crystal molecules are aligned using directional characteristics because the liquid crystal molecules have long and thin shapes. In this respect, an electric field is applied to the liquid crystal for controlling the alignment direction of the liquid crystal molecules. That is, if the alignment direction of the liquid crystal molecules is controlled by the electric field, the light is polarized and changed by the optical anisotropy of the liquid crystal, thereby displaying the picture image.
The liquid crystal is classified into positive (+) type liquid crystal having positive dielectric anisotropy and negative (−) type liquid crystal having negative dielectric anisotropy according to electrical characteristics of the liquid crystal. In the positive (+) type liquid crystal, a longitudinal axis of a positive (+) liquid crystal molecule is in parallel to the electric field applied to the liquid crystal. Meanwhile, in the negative (−) type liquid crystal, a longitudinal axis of a negative (−) liquid crystal molecule is perpendicular to the electric field applied to the liquid crystal.
FIG. 1 is an exploded perspective view illustrating some parts of a general twisted nematic (TN) mode LCD device. As shown in FIG. 1, the general TN mode LCD device includes lower and upper substrates 1 and 2 bonded to each other at a predetermined interval, and a liquid crystal layer 3 injected between the lower and upper substrates 1 and 2.
More specifically, the lower substrate 1 includes a plurality of gate lines 4, a plurality of data lines 5, a plurality of pixel electrodes 6, and a plurality of thin film transistors T. The plurality of gate lines 4 are formed on the lower substrate 1 in one direction at fixed intervals, and then the plurality of data lines 5 are formed perpendicular to the plurality of gate lines 4 at fixed intervals, thereby defining a plurality of pixel regions P. Subsequently, the plurality of pixel electrodes 6 are respectively formed in the pixel regions P defined by the plurality of gate and data lines 4 and 5 crossing each other, and the plurality of thin film transistors T are respectively formed at crossing points of the plurality of gate and data lines 4 and 5. Also, the upper substrate 2 includes a black matrix layer 7 that excludes light from regions except the pixel regions P, R/G/B color filter layer 8 for displaying various colors, and a common electrode 9 for displaying a picture image.
The common electrode 9 is formed on the upper substrate 2, so that a common line is formed in a dummy region of the lower substrate 1 in order to apply a common voltage to the common electrode 9, and Ag dots are formed between the upper and lower substrates for electrically connecting the common line and the common electrode 9 to each other. The aforementioned structure will be described in detail. FIG. 2 is a plan view illustrating a general TN mode LCD device, and FIG. 3 is a cross-sectional view taken along line I-I′ of FIG. 2.
As mentioned above, FIG. 2 is a layout of lower and upper substrates 1 and 2 bonded to each other at a predetermined interval. The respective lower and upper substrates 1 and 2 have active and dummy regions. In the active region of the lower substrate 1, as shown in FIG. 1, there are the gate line 4, the data line 5, the pixel electrode 6 and the thin film transistor T. Meanwhile, as shown in FIG. 2, the dummy region of the lower substrate 1 includes the common line 100 for applying the common voltage to the common electrode 9 of the upper substrate 2, and outgoing lines 103 and 104 respectively connecting the gate and data lines 4 and 5 to gate and data driving ICs 101 and 102. Also, as shown in FIG. 1, the active region of the upper substrate 2 includes the black matrix layer 7, the R/G/B color filter layer 8 and the common electrode 9, and the dummy region of the upper substrate 2 includes the black matrix layer (106 of FIG. 3).
After that, a sealant 105 is formed in the dummy region between the upper and lower substrates 1 and 2, thereby bonding the upper and lower substrates 1 and 2 to each other. Then, the liquid crystal layer 3 is formed between the upper and lower substrates 1 and 2. At this time, unexplained reference numbers 107 and 108 are alignment layers, and an unexplained reference number 109 is a spacer. In this case, the sealant 105 is a heat-hardening sealant, and a pattern of the sealant 105 has a liquid crystal injection inlet. Accordingly, after bonding the upper and lower substrates, the sealant is hardened in Hot Press, and the liquid crystal is injected into a space between the upper and lower substrates through the liquid crystal injection inlet in a vacuum chamber.
In the aforementioned TN mode LCD device, the liquid crystal layer 3 positioned on the pixel electrode 6 is aligned according to a signal applied from the thin film transistor T, and light transmittance transmitting the liquid crystal layer 3 is controlled by the alignment of the liquid crystal layer 3, thereby displaying the picture image. Also, in the aforementioned LCD device, liquid crystal molecules are driven according to an electric field formed at upper and lower sides between the common electrode 9 and the pixel electrode 6, to be perpendicular to the substrates, thereby obtaining great transmissivity and high aperture ratio. Also, the common electrode 9 of the upper substrate 2 serves as a ground, whereby it is possible to prevent liquid crystal cells from being damaged by static electricity. However, it is not advantageous because it is hard to obtain a wide viewing angle.
In order to solve this problem, an IPS mode LCD device is proposed. FIG. 4 is a plan view illustrating an IPS mode LCD device according to the related art, and FIG. 5 is a cross-sectional view taken along line II-II′ of FIG. 4.
As shown in FIG. 4 and FIG. 5, a plurality of gate lines 22 are arranged at fixed intervals in one direction on an active region of a transparent lower substrate 21, and a plurality of data lines 25 are formed perpendicular to the gate lines 22 at fixed intervals, thereby defining a plurality of pixel regions P. Then, a common line 29 is formed in the pixel region P in parallel to the gate line 22, and a thin film transistor T is formed in each pixel region P defined at the crossing of the gate and data lines 22 and 25. The thin film transistor T includes a gate electrode 22a projecting from the gate line 22, a gate insulating layer on an entire surface of the transparent lower substrate 21, an active layer 24 on the gate insulating layer 23 above the gate electrode 22a, a source electrode 25a projecting from the data line 25, and a drain electrode 25b at a fixed interval from the source electrode 25a. 
In the respective pixel regions P, a plurality of pixel electrodes 28 are formed parallel to the respective data lines 25 at fixed intervals from the respective data lines 25. One end of the pixel electrode 28 is connected to the drain electrode 25b of the thin film transistor T. Also, in the pixel region P, a plurality of common electrodes 29a are projected from the common line 29. The pixel electrode 28 is formed parallel to the common electrode 29a. After that, a passivation layer 26 is formed on the entire surface of the transparent lower substrate 21. At this time, the passivation layer 26 is formed of SiNx or SiOx. Then, an alignment layer (not shown) is formed of polyimide on the passivation layer 26.
Meanwhile, a transparent upper substrate 31 is formed to being opposite to the transparent lower substrate 21. The transparent upper substrate 31 includes a black matrix layer 32 for preventing light leakage, R/G/B color filter layer 33 for displaying various colors, and an overcoat layer 34. Then, a liquid crystal layer 35 is formed between the transparent lower and upper substrates 21 and 31. Next, a plurality of spacers 36 having a predetermined size are formed between the transparent lower and upper substrates 21 and 31 to maintain a fixed interval therebetween. As mentioned above, the IPS mode LCD device forms the common electrode 29a and the common line 29 on the active region of the transparent lower substrate 21. In this respect, the IPS mode LCD device is different from the TN mode LCD device in that the common line is not formed in the dummy region of the transparent lower substrate 21.
FIG. 6 is a cross-sectional view illustrating a sealant pattern in the dummy region of the IPS mode LCD device according to the related art. As shown in FIG. 6, a sealant 30 is formed in the dummy region between the transparent lower and upper substrates 21 and 31, thereby bonding the transparent lower and upper substrates 21 and 31 to each other. At this time, the common line is not formed in the dummy region of the transparent lower substrate 21 having the sealant 30 thereon. Also, the black matrix layer 32 is formed in the dummy region of the transparent upper substrate 31., Herein, unexplained reference numbers are the overcoat layer 34 and the liquid crystal layer 35. The sealant 30 is a heat-hardening sealant, and a pattern of the sealant 30 has a liquid crystal injection inlet. In general, the heat-hardening sealant is formed of epoxy resin, urethane resin or phenol resin. Especially, the epoxy resin is most generally used for the heat-hardening sealant.
In the case of using the epoxy resin for the heat-hardening sealant, an epoxy ring is opened by a hardener such as amine or amide, and then the epoxy ring is reactive on another epoxy ring, whereby the epoxy rings are sequentially opened, thereby forming a high molecular chain. This process is referred to as hardening. The epoxy resin is classified into a normal temperature hardening type and a heat-hardening type. The normal temperature hardening type epoxy resin is hardened at a normal temperature, and the heat-hardening epoxy resin is hardened by performing a heat process between 120° C. and 140° C. for 30 minutes to 1 hour. For completing this process, the heating process is generally performed. That is, the two substrates are completely bonded to each other by the hardened epoxy resin. The hardened epoxy resin has great supporting characteristics, and hardness. Accordingly, after bonding the two substrates, the heat-hardening sealant is hardened in Hot Press, and the liquid crystal is injected between the two substrates through the liquid crystal injection inlet in a vacuum chamber.
However, the method for manufacturing the LCD device, in which the liquid crystal is injected between the two substrates after bonding the two substrates with the heat-hardening sealant, has the following disadvantages.
Accordingly, as a size of an LCD panel increases, much time is spent injecting the liquid crystal between the two substrates, and may generate failures in injecting the liquid crystal completely.
Since the heat-hardening sealant is used for the sealant, it generates thermal expansion. Also, when hardening the sealant, the sealant may leak out, thereby contaminating the liquid crystal, and generating spots on the LCD panel. That is, the heat process is performed at 250° C. between 2 and 3 hours to completely harden the epoxy resin of the heat-hardening sealant. Thus, before hardening the epoxy resin completely, the liquid crystal may flow into the active region, whereby the liquid crystal may be contaminated, and the spots may be generated on the LCD panel.